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Modelling Laurentide ice stream thermomechanics Marshall , Shawn Joseph


Ice streams are fast-flow currents which represent a small areal fraction in an ice sheet but account for the majority of ice sheet drainage. Because ice streams are inherently complex and are subgrid in current numerical models, they have not been portrayed in large-scale ice sheet studies. I employ a continuum mixture framework to incorporate ice streams in a three-dimensional thermomechanical model of the Laurentide Ice Sheet. The ice mass is composed of a binary mixture of sheet ice, which flows by viscous creep deformation, and stream ice, which flows by decoupled sliding and/or sediment deformation at the bed. Dynamic and thermal evolutions are solved for each component in the mixture, with coupling rules to govern transfer between flow regimes. These transfers represent the activation, growth, and deactivation of ice streams, manifest by creep exchange and bed exchange of ice. I express the governing equations for mass, momentum, and energy balance in a form suitable for direct incorporation in existing numerical models of ice thermomechanics. The model is applied in simulations of an ice stream draining Hudson Strait and a large-scale surge lobe on the southern Laurentide margin. Ice stream vivacity is determined by the relative magnitudes of gravitational driving stress, longitudinal stress, marginal shear stress, and basal drag in the momentum balance. Sensitivity tests of poorly-constrained parameters in this stress balance permit a wide range of stream demeanors. Under simple thermal regulation of stream activation, internal flow oscillations arise over the physically-reasonable range of most free parameters. While simulated surge periodicities vary widely, bounds can be placed on meltwater and iceberg flux from Hudson Strait during a surge event. Through detailed characterization of the Laurentide bed, I compile subgrid terrain attributes of relevance to ice dynamics and explore geologic and topographic influences on fast flow. I also introduce a bed thermal model which tracks permafrost evolution, refining the calculation of basal energy balance which is integral to thermal facilitation of fast flow. Collectively, this work represents initial steps toward an improved description of subglacial conditions, ice stream mechanics, and fast-flow instabilities in a large-scale ice sheet model.

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